1. Field of the Invention
L-amino acid fermentation liquors contain large quantities of microbial cells and soluble proteins, which must be removed because they retard the growth of amino acid crystals and otherwise negatively affect efficient recovery of the amino acid product.
A great variety of other impurities (inorganic salts, sugar, pigments, etc.) derived from the culture medium used and from the metabolism of the microorganisms employed are also contained in the fermentation liquor, and these must also be removed by sophisticated combinations of various isolation and purification techniques, such as ion exchange, activated charcoal treatment, and crystallization.
In the past, isolation of amino acids from fermentation liquors containing them and purification thereof, have been performed as follows: microbial cells and other insoluble impurities are first removed by centrifugal separation, filtration, coagulation or sedimentation, the pH of the resulting solution is adjusted so that the amino acid being purified will be in cationic form, and the cationic amino acid thus formed is adsorbed onto a strongly acidic cation-exchange resin. The adsorbed amino acid is eluted with a dilute alkali solution, the eluate is decolorized with activated charcoal, and the free amino acid or salt thereof is separated in crystalline form by concentration, cooling or neutralization. The crystals thus obtained may be further purified, as required, through recrystallization. A large number of amino acids are produced based on this process on an industrial scale, including arginine, glutamine, histidine, isoleucine, lysine, proline, threonine, serine and valine.
In industrial operations, centrifugal settlers, nozzle-discharge type, continuous centrifugal separators and basket-type centrifugal separators are frequently used for removal of microbial cells and other insoluble impurities, while vacuum and press filters using a precoat of diatomaceous earth are principally employed for filtration.
However, it is difficult to completely remove microbial cells by centrifugal force, and filtration is unable to remove soluble proteins. This gives rise to various problems in the succeeding steps: clogging of columns packed with ion-exchange resin, retarded growth of amino acid crystals, coagulation of denatured proteins during heating and concentration, which contaminate the crystals of amino acid and significantly lower its purity, and others. The result is that ion-exchange resin treatment is not as effective as it could be, and recrystallization has to be conducted repeatedly to obtain crystals with the desired purity.
On the other hand, anionic substances, pigments and polymers contained as impurities are also responsible for retardation of crystal growth, as well as lowered purity and discoloration of separated crystals. Use of anion-exchange resin, amphoteric ion-exchange resin or adsorbent synthetic resin have been adopted to remove such impurities. In this case, too, the above-mentioned soluble proteins tend to be adsorbed on these resins, thus markedly diminishing their actions and often making their regeneration impossible as a result of irreversible adsorption.
In view of the above drawbacks of the prior art methods, there remains a need for new and improved methods for recovering high purity amino acids from fermentation liquors.